1. Field of the Invention
The invention relates to the field of metallurgy. The invention relates to the field of a direct current electric arc furnace.
2. Description of the Related Art
Formerly alternating current electric arc furnaces have mainly been utilized for electric furnaces wherein the melting and refining of metals are carried out.
Recently the electric arc furnace is being switched from an alternating current type to a direct current type since the capacity of semiconductors for power supply is enhanced and the cost thereof is lowered. The direct current electric arc furnace is superior to the alternating electric current arc furnace in the efficiency of power supply, the consumption of electrodes of the furnace and the consumption of electricity.
The feeding system of the direct current electric arc furnace is the same with that of the alternating current electric arc furnace in power supply to transformers. The voltage is lowered by the transformers and the alternating current is rectified to the direct current by a rectifier such as a thyrister. In the rectifying system of the thyrister direct current reactors are attached to the direct current circuit to prevent rapid increase of electric current when the system is shortcircuited. This direct current system is composed of an anode side conductor which leads to a bottom electrode of the furnace and a cathode side conductor which leads to a movable carbon electrode of the furnace.
The rectifier circuit rectifies the alternating current by dividing the current to 2 or 4, which constitutes 12 or 24 phased current respectively, to prevent higher harmonics hazard. The rectified currents are connected to the conductors in parallel connection.
In case of the alternating current electric arc furnace three elecrodes are used wherein three phase alternating current is supplied to the electrodes.
In case of the direct current electric arc furnace a single movable electrode can be used so far as the current is in the range allowable for the movable electrode. Therefore the facility around the movable electrode is simple but a bottom electrode is required. In the direct current electric arc furnace, by placing an electrode at the center of the furnace, uniform melting is carried out which decreases the consumption of electricity and refractories.
FIG. 6 is an explanatory view of the feeding system of a conventional direct current arc furnace. As shown in FIG. 6 the electricity supplied from a main supply is fed to the transformer 3 via the disconnecting switch 1 and the circuit breaker 2. This is the case wherein 24-phase alternating current is rectified utilizing four sets of the tansformer 3 which lowers the voltage. The secondary side terminals of the transformer 3 are connected to the input terminals of the phase selectable thyrister, or the thyrister 4. The minus side terminals of the thyrister 4 are connected to the moving electrode 7 via the feeding conductor 6 and the plus side terminals thereof are connected to the bottom electrode 9 via the direct current reactor 5 and the feeding conductor 8. The holder arm 10 holds the movable electrode 7 and moves it up and down. The main body 12 contains the steel bath 11.
In this conventional direct current electric arc furnaces, when the capacity of the furnace is enhanced, the feeding conductors 6 and 8 should be arranged at the place wherein the cost of the installation of these conductors is decreased or at the place where the conductor 8 does not interfere with the cooling facility attached in the vicinity of the bottom electrode 9.
In this arrangement the magnetic field caused by the electric current flowing in the conductors 6 and 8 and the magnetic field caused by the arc under the moving electrode 7 interfere with each other which gives rise to the deviation of the arc. In this case the deviation is towards the feeding facility.
Accordingly the reaction in the furnace is accelerated at the deviated side and retarded at the opposite side. Hence the uniform reaction is not attainable. The deviated distance from under the arc is proportional to the intensity of the arc current. When the arc current is over 40 kiloampere, the deviated distance is significant. It is very difficult to arrange the bottom electrode 9 and the feeding conductors 6 and 8 symmetrically with respect to the horizontal and the vertical position. Therefore due to the deviation of the arc the uniform melting is not possible.